Method for glass to glass sealing uttlizing softened and rigid circumferential segments



March 31, 1970 H. c. WERNER 3,503,727

METHOD FOR GLASS TO GLASS SEALING UTILIZING SOFTENED AND RIGID CIRCUMFERENTIAL SEGMENTS Filed Dec. 12, 1967 Aliza/wed United States Patent METHOD FOR GLASS TO GLASS SEALING UTILIZING SOFTENED AND RIGID CIR- CUMFERENTIAL SEGMENTS Herbert C. Werner, Lancaster, Pa., assignor to RCA Corporation, a corporation of Delaware Filed Dec. 12, 1967, Ser. No. 689,904 Int. Cl. C03b 29/02; C03c 27/06 US. Cl. 65-57 9 Claims ABSTRACT OF THE DISCLOSURE A method of hermetically sealing together the stem and bulb of a glass envelope of an electron tube includes closely fitting the cylindrical stem concentrically inside the open end of the bulb so that the bulb wall overlaps the stem wall in a telescoping fashion. The bulb, with stem in place, is then seated on a vacuum system and evacuated. When the pressure inside the bulb is reduced to a value considerably below normal atmospheric pressure, first circumferential segments of an annular area of the overlapping part of the bulb wall are successively heated to softening temperature so that. the gas pressure on the outside of the bulb wall collapses the wall inwardly to contact and fuse with the stem wall. Throughout the heating, second circumferential segments of the annular area remain cool and rigid, so that there is no change in overall dimensions of the bulb.

BACKGROUND OF THE INVENTION Field of the invention My invention relates to the art of joining two lengths of glass tubing by heating, and particularly concerns hermetically sealing together a stern and a bulb to form a glass envelope of an electron tube.

Description of the prior art In the art of manufacturing glass envelopes for electron tubes, it is sometimes desirable to seal two sections of a glass envelope together while they are evacuated in their interior. One kind of electron tube that is manufactured in this manner is a particular kind of vidicon camera tube. This tube has a bulb section which is essentially a right circular hollow glass cylinder having a transparent faceplate at one end and an opening at the other end. The tube also has a stem with an open ended portion which is also a right circular hollow glass cylinder but with an outside diameter slightly smaller than the inside diameter of the bulb. At the inside surface of the bulb faceplate is a photoconductive layer which is easily damaged by such gases as are found in the normal atmosphere. Therefore, the bulb interior should be kept under vacuum even while the bulb is being sealed to a stem. One way this may be accomplished is to insert the open portion of the stem into the lower, open, portion of the bulb in a telescoping fashion with walls overlapping. The open end of the bulb is then seated on a vacuum system, and the bulb with stem partly inside, is evacuated. When the pressure inside the bulb is considerably below normal atmospheric pressure, the entire circumference of an annular portion of the overlapping bulb wall is heated to a working temperature of about 1000 C. As the heated bulb wall becomes soft, it collapses inwardly to contact the stem wall and by that contact to heat it until it is also soft and fuses with the bulb wall to make a tight seal. One difiiculty associated with this method is encountered when it is important that the bulb not change its overall dimensions. For instance, a vidicon camera tube may have attached to the stem thereof some components, such as an electron gun or grid, which should 3,503,727 Patented Mar. 31, 1970 be very precisely positioned with respect to the photoconductive layer on the faceplate and aligned with respect to the axis of the bulb. Should the bulb shrink in length or be misaligned from its axis as a result of the sealing procedure, the desired alignment of the component would be destroyed. When the entire annular portion of the overlap is heated simultaneously to softening temperature, the upper portion of the bulb is no longer rigidly connected to the lower portion and is thus easily turned off the original bulb axis by gravity, air pressure, or by uneven cooling of the heated glass at the joint. Moreover, shrinking of the glass that was heated may shorten the total length of the bulb somewhat. Such shrinkage or deformation which .results in a change of the over-all dimensions of the bulb,

can be prevented by mechanically holding in place the upper portion of the bulb while the joint is soft and by allowing for shrinkage when making the original alignment of stem and bulb. These measures, however, require additional complexity in labor and hardware used in the manufacture of such tubes.

SUMMARY OF THE INVENTION A method of sealing a cylindrical portion of a glass stem to a cylindrical portion of a glass bulb having an open end comprises first positioning the stern inside the open end portion of the bulb in a telescoping fashion. The bulb interior is then evacuated. Small circumferential segments of an annular area of the overlapping portion of the bulb wall are then heated successively until they be come softened and collapse inwardly to contact and fuse with the stem wall. Circumferential segments adjacent to a heated segment remain cool enough so that they rigidly support the bulb to preserve the original overall dimensions of the bulb and stem.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view of an evacuating and sealing apparatus with loosely assembled glass envelope parts of a vidicon camera tube mounted thereon.

FIG. 2 is a sectional view of the lower portion of a sealed vidicon camera tube.

. DESCRIPTION OF THE PREFERRED EMBODIMENT One form of apparatus used in practicing the method described herein is that designed for sealing together two sections of a glass envelope for a vidicon television camera tube.

As shown in assembled but unsealed condition in FIG. 1, the vidicon tube 10 has a bulb 12 portion and a stem portion 14. The bulb portion 12 may be a length of borosilicate Coming 7052 glass tubing about 6 inches long, 78 inch inside diameter and 1 inch outside diameter. One end of the bulb 12 is closed by a faceplate 16, at the inside of which is a photoconductive layer 18 that is readily damaged by exposure to normal atmosphere. The stem portion 14 is also a length of borosilicate Corning 7052 glass tubing and is about 1 inch long, inch inside diameter and slightly less than inch outside diameter. The stem has embedded in it a series of electrical leads 22 made of Kovar, a metal alloy composed of 29% nickle, 17% cobalt, 0.3% manganese and 53.7% iron. Sealed to one end of the stem 14 is a length of about inch inside diameter and A inch outside diameter glass tip off tubing 24 to allow vacuum sealing of the entire sealed envelope after the stem 14 and bulb 12 have been sealed together. Mounted to the inside parts of the Kovar leads 22 and spaced at a short distance from the photoconductive coating 18 is an electron gun device 26.

During a sealing operation, the lower portion of the bulb 12 is fitted over the open end of the stem 14 so that the electron gun device 26 is properly spaced from the photoconductive layer 1-8. The assembled vidicon tube is then placed as shown in FIG. 1 on a base plate 28 so that the open end of the bulb 12 is seated on a gasket 30 about an opening 31 in the baseplate leading to vacuum pumping equipment not shown. Resting on the top of the baseplate 28 is a torch 32 so constructed that it may be readily moved about the vidicon 10 in a circular fashion. The torch 32 shown in FIG. 1 comprises a gas manifold 34 in the form of a length of hollow tubing bent to a circular shape with ends joined and standing on three attached legs 36 that are fitted at their bottom to a circular groove 37 in the top of the baseplate 28. At the outside perimeter of the manifold 34 is attached a handle 38 to facilitate rotation of the manifold. At a point opposite the handle 38 and also on the outside perimeter of the manifold 34 is attached a short length of tubing 39 providing access to the interior of the manifold 34 and acting as an inlet for a mixture of air and natural gas or city gas. At the inside perimeter of the manifold 34 and spaced inch from the bulb 12 is attached a torch burner tip 40 of a type commonly used for burning a mixture of natural gas and air. The burner tip 40 is so constructed and the gas and air mixture supplied to it from the manifold 34 is so regulated that the visible flame issuing from the burner tip is about inch long and the blue cones in the interior of the flame are about 75 inch long. Moreover, the height of the flame is such that it strikes the bulb 12 at points on an annular area about the bulb 12 between about inch to about inch below the open end of the stem 14.

After a vacuum of approximately 10- torr has been drawn on the vidicon tube 10 assembly, the torch 32 is ignited and moved around the bulb 12 only once at a constant rate of about one revolution per minute with about a inch overlap of final position over starting position. As the torch 32 moves along the annular area or seal region of the bulb 12 with which its flame will come in contact during its travel about the bulb 12, each small circumferential segment of the seal region which the flame contacts is heated to about 900 C., that is, a temperature above the softening temperature but below the Working temperature of the glass. As this temperature is approached, the atmospheric pressure at the outside of the bulb 12 collapses, or crimps, the wall of the softened bulb 12 inwardly until it contacts the wall of the stem 14. With this contact enough heat is conducted to the stem 14 wall to allow it to also soften and collapse inwardly slightly, at a temperature of about 800 C. and at the same time to form a strong fused bond to the bulb 12 wall. At this time the torch 32 is moved away to an adjacent segment of the seal region and the previously bonded segment is allowed to cool. Since the burner tip 40 only heats a small segment of the seal region to softening temperature at any one time, the remaining segments of the seal region remain sufficiently rigid to preserve the original overall dimensions of the bulb and stem. The total length of the bulb and stem thus remains essentially the same since the upper and lower portions, above and below the seal region, do not change positions relative to one another. Hence, as the torch 32 makes its full circle with the inch overlap and forms a gas tight seal 42 between stem 14 and bulb 12 as shown in FIG. 2, the spacing between the electron gun 26 and the photoconductive layer 18 remains precisely what it was on assembly prior to scaling, without the necessity of holding in place the upper part of the bulb 12 during the sealing operation.

The method herein described may be used for joining pieces composed of ordinary hard glass such as Corning 70 52 glass. This is contrary to normal expectation that the uneven heating involved in a practice of the instant method would cause fracturing when applied to ordinary hard glass as opposed to quartz glass or to Pyrex brand glass known by the glass code number Coming 7040. The aspect of my method which allows its application to a successful sealing of 7052 glass is a highly desirable one. One advantage of using 7052 glass is that it may be Worked with only a natural gas and air flame rather than a hydrogen and oxygen flame. Consequently, in practicing my method it is not necessary to have available separate means for producing an annealing flame, for an annealing flame is usually also a natural gas-air flame with an increased amount of gas. Another advantage is that 7052 glass seals well to Kovar leads, to form exceptionally strong and dependable electrical leads to internal components of a glass envelope. These advantages and the fact that 7052 glass is widely used throughout the conversion tube industry and therefore is readily available in various preformed shapes contribute the substantial saving in production cost.

I claim:

1. A method of sealing a cylindrical hollow glass stem to an inner wall of a cylindrical, hollow glass bulb having an open end comprising:

(a) placing said stem in coaxial relation with and at least partially inside of said bulb to form an annular area of said bulb Where said bulb overlaps said stem defining a periphery of said bulb,

(b) evacuating the space between said stern and said annular area of said bulb,

(c) successively heating mutually adjacent first circumferential segments of said annular area of said bulb to a sealing temperature and fusing said first segments to said stem while leaving second circumferential segments of said annular area relatively cool and rigid, and

(d) maintaining said bulb and stem in their relative positions with respect to one another during said heating.

2. A method of sealing a glass stem to an open end portion of a tubular glass bulb, wherein said glass stem has a smaller diameter than the inner diameter of said bulb, comprising:

(a) inserting said stem into said end portion in coaxial relation with respect to said bulb, to form an annular space between said stem and the inner wall of said bulb,

(b) closing the open end of said bulb, to form an hermetic enclosure,

(c) evacuating said enclosure, and

(d) heating successive first circumferential portions to softening temperature while leaving adjacent second circumferential portions relatively cool and rigid of an annular region of said bulb in radial register with said stem for successively inwardly collapsing such successive portions across said annular space and fusing into sealing contact with said stem.

3. A method of hermetically sealing together a glass bulb and a stem of.a hard glass electronic tube envelope, said bulb being a right circular cylinder having a hollow interior with a predetermined diameter and an open end, said stern having a wall in the form of a right circular hollow cylinder with an outside diameter slightly less than said predetermined inside diameter of said bulb, said method comprising:

(a) positioning said stem inside said bulb in a telescoping fashion and in an annular position relative to said bulb,

(b) seating said open end of said bulb on resilient gasket means for providing an hermetically tight enclosure when evacuating said hollow interior of said bulb and for preserving said predetermined position of said stem,

=(c) evacuating said interior of said bulb to a pressure substantially below normal atmospheric pressure, and

:(d) heating only successively first circumferential segments of a band of said bulb wall in radial register with said stern wall to at least the softening temperature of said bulb wall so that said bulb wall Sections when heated successively collapse toward and make fusing contact with said stem wall about the entire said band, and

(e) simultaneously cooling second circumferential segments of said band adjacent to said heated segments to a temperature below said softening temperature, so that said second circumferential segments are relatively cool and rigid.

4. A method of hermetically sealing a circular, cylindrical, hollow, borosilicate glass bulb portion of a vidicon camera tube envelope having a wall and an open end to a circular, cylindrical, hollow, Kovar-sealing borosilicate glass stem portion of said envelope, comprising:

(a) locating said bulb and said stein in a telescoping relation to one another with said bulb wall overlapping said stem, to form an annular space between said bulb wall and stem,

(b) evacuating said annular space between said overlapping bulb wall and said stem to a pressure considerably below normal atmospheric pressure,

(c) heating successively first circumferential segments of an annular section of said bulb wall in register with said annular space to at least the softening temperature of said wall while leaving second circumferential segments sufficiently cool and rigid to preserve the original overall dimensions of said bulb and said stem, and

(d) fusing said first segments with said stem.

5. A method of hermetically sealing a hollow, cylindrical, Kovar-sealing borosilicate glass portion of a stem to a hollow cylindrical glass portion of a bulb to form an envelope comprising:

(a) locating said bulb and said stem in a telescoping relation to one another with said stem. wall overlapping said bulb and defining an annular space therebetween,

(b) evacuating at least an annular section of said space between said overlapping stem wall and said bulb to a pressure considerably below normal atmospheric pressure,

(c) heating and fusing successively to said bulb first circumferential segments of an annular section of said stem wall in radial register with said annular section of space to at least a softening temperature of said wall, while leaving second circumferential segments adjacent to the heated segments sufficiently cool and rigid to preserve the original overall dimensions of said bulb and said stem.

6. Method of sealing a circular, Kovar-sealing borosilicate glass member to the inner wall of a hollow cylindrical glass structure wherein said glass member has a smaller diameter than the inner diameter of said glass structure, comprising:

(a) placing said glass member coaxially within said glass structure,

(b) evacuating the interior of said glass structure in a region adjacent to and surrounding said glass member, and

(c) moving only one heat source around said glass structure in a path adjacent to said region at a given angular velocity for successively softening and fusing with said glass member a plurality of first circumferential segments defining a first part of the periphery of said glass structure in said region while preserving a substantial number of second circumferential segments defining a second part of said periphery below the softening temperature of the glass of said structure, so that said second circumferential segments are relatively cool and rigid.

7. Method according to claim 6 and wherein said heat source is moved angularly with respect to said glas structure at a linear velocity of about 1r inches per minute relative to said periphery.

8. Method of sealing a circular, Kovar-sealing borosilicate glass member to the inner wall of a hollow glass cylindrical structure comprising:

(a) positioning said glass member within and in coaxial and close spaced relation with respect to said structure to form a region of radial register,

(b) evacuating the space between said structure and said glass member, and

(c) passing a relatively small area heat source around a periphery of said glass structure adjacent to the region of radial register of said glass member with said glass structure in substantially one pass, to heat relatively small first circumferential segments of said glass structure at said region successively to sealing temperature and into fusion with said member while second circumferential segments are cooled for preserving the initial axial dimension of said glass structure.

9. The method defined in claim 1 and wherein said heating step comprises moving a small area heating source about said annular area at a relatively constant rate of about 11' inches per minute relative to said periphery of said annular area of said bulb.

References Cited UNITED STATES PATENTS 12/1948 Ferrell --59 6/1954 Fraser 65-57 Dedication 3,503,727.-Herbert 0. Wemer, Lancaster, Pa. METHOD FOR GLASS TO GLASS SEALING UTILIZING SOFTEN ED AND RIGID CIR- CUMFERENTIAL SEGMENTS. Patent dated Mar. 31, 1970. Dedication filed June 14, 1974, by the assignee, BOA Corpowztion. Hereby dedicates the entire remaining term of said patent to the Public.

[Ofiicz'al Gazette November 26, 1.974.] 

